1. Field of the Invention
The present invention relates to image forming devices such as xerographic printing and copying machines, and in particular, a device and method for preventing image size distortion and color misregistration of images. The device and method of the present invention compensate for changes in tension in an intermediate image carrying belt which causes the belt to stretch or shrink resulting in image distortion and color misregistration.
2. Relevant Art
Designers of xerographic printers and copiers have generated several solutions to the problems of image size distortion and color misregistration of toned images formed on an intermediate image carrying belt. The intermediate belt; made of a dielectric material, serves as an image carrier. Tension in the intermediate belt varies according to changes in several factors including the contact force between the belt and photoreceptive drums and belt drive rollers, differences in rotating speed of the belt and photoreceptive drums, and misalignment of the belt and photoreceptive drums.
In a monochrome copying mode, stretching of the belt produces an image larger than the original image and shrinking of the belt produces an image smaller than the original image. The amount of change in the size of the image produced depends on the amount of belt stretching or shrinking which varies according to changes in belt tension. In a polychrome copying mode, not only are the images enlarged or reduced as in the monochrome mode, but the images are also subject to color misregistration.
One solution to the above problems is to use a stiffer and thicker belt that is far less susceptible to stretching or shrinking However, the stiffer and thicker belts, made of materials such as stainless steel, more readily propagate motion errors such as those caused by vibration of the belt. These types of motion errors and others are highly detrimental to the image forming process. Thus, the stiffer metal belts are not that desirable.
Another solution to correct image distortion is slip transfer. In polychrome systems, the color registration errors accumulate because of the imperfections in the size and shape of the mechanical parts. To overcome the above problems, a slip transfer is implemented so that the photoreceptive drums are rotated at a speed slightly faster than the rotating speed of the intermediate transfer belt. However, slip transfer can only prevent a limited amount of misregistration and creates additional problems with image smearing, Also, the amount of slip transfer is difficult to control when high-pressure biased transfer is used to transfer images from the photoreceptive drums to the intermediate belt.